Four unknown compounds supplied by manufacturers were tested in primates in the search for promising antimicrobial agents to prevent dental plaque formation and caries activity. Changes in the population of dental plaque organisms were examined by using Flourescent Antibody (FA) reagents specific for S. mutans, S. sanguis and four species of Actinomyces. An increase in the proportion of S. sanquis and a reduction of Actinomyces was observed with decreasing plaque scores. For two of the agents this effect was equal to or more pronounced than that obtained with chlorhexidine which served as a control.